The present invention generally relates to a method for the thermal characterization of packaging materials used to contain semiconductor devices, and more particularly to the use of a platinum resistor test unit designed to accomplish such characterization in a fast and efficient manner.
Semiconductor devices, including integrated circuits, are traditionally placed within packages designed to secure and contain the devices during operation. Typical packages include conventional DIP (dual-in-line) packages made of plastic, ceramic packages (CERDIP), hybrid units, metal can-type units, and flat pack small geometry units.
In determining the operational capabilities of a semiconductor device contained within a package, it is important to calculate the amount of heat the device will generate, and how effectively the heat is dissipated through the package. The life span of a semiconductor device is directly related to its operational temperature. It is therefore important to first determine the junction temperature (T.sub.j) of the device during operation within its package. The "junction temperature" is defined as the temperature at the surface of the device while it is operating. Temperature extremes as small as 10.degree. C. above the T.sub.j determined to be "critical" for a particular semiconductor device can reduce its operational lifetime by over one-half. Junction temperature values over 160.degree. C. will almost always result in very early failure of the device.
Next, it is necessary in any semiconductor packaging system to determine the rate at which heat will flow from the semiconductor device out of its package. This heat flow is traditionally expressed in .degree.C./watt. Having accurate information regarding the heat dissipation of a semiconductor device in its native environment (e.g. within its package) facilitates the manufacture of improved packaging systems and heat sink units associated therewith.
Traditionally, two methods have been used to thermally characterize semiconductors and their packaging systems. A general discussion of such methods may be found in the article "Semiconductor Thermal Considerations in System Design" by Siegel (December 1983, Sage Enterprises offprint article). The first of these methods involves the use of an infrared microscope system (IR method). The IR method uses an infrared detector coupled with an optical microscope. To characterize a semiconductor device within a package, the surface of the device must first be exposed. Using the optical microscope and attached infrared detector, the device can be scanned to generate an average temperature profile, or attention can be directed to individual points on the device.
However, there are certain disadvantages inherent in the IR method. First, the package containing the semiconductor device must be partially opened so that the surface of the device can be viewed. This prevents an accurate determination of the heat dissipation from the device in its native, packaged environment since a portion of the package and any heat sinks attached thereto must be removed.
In addition, the semiconductor device being tested must, in most cases, be coated with a special material having a constant emissivity. Such coating can alter the heat dissipation characteristics of the device, making accurate thermal analysis even more difficult. Other disadvantages of the IR method include high equipment and maintenance costs, and limited accuracy.
Another frequently used method is called the Pulsed Diode/Transistor V.sub.be method (PDT method). Most semiconductor devices have at least one temperature-dependent characteristic. In a transistor, the base to emitter voltage (V.sub.be) is temperature-dependent. In a diode, the forward voltage (V.sub.f) is temperature-dependent. The temperature-dependent characteristics of these devices may be plotted against externally-applied temperatures to yield a graph. The devices are then inserted within a selected package in place of the semiconductor for which it was designed, and a test simulation is run. By using data obtained from the simulation in conjunction with the previously-prepared graph, the thermal characteristics of the package can be determined under various operational conditions.
However, there are also numerous disadvantages inherent in the PDT method. First, it requires fairly expensive test equipment and is slow to use. In addition, it is not very accurate since the test device (transistor, diode, etc.) is a "point source" and does not consider the overall size of the semiconductor device for which the package was designed.
The present invention represents a substantial departure from the above-described methods in numerous aspects. Primarily, it avoids the use of expensive test equipment, while increasing the speed, accuracy, and ease with which the thermal characteristics of a semiconductor packaging system may be determined.